Recent evidence indicates that accessible hydrophobic groups occur much more frequently on the surface of protein molecules than had been surmised. Subsequent results showed that the hydrophobic factor can be used as a new and independent parameter for the separation of proteins through the use of hydrophobic adsorbents in the presence of high concentrations of a "structure forming" salt. The salt not only enhances hydrophobic "bonding" but simultaneously quenches electrostatic interaction. Separation is more effectively achieved through differential adsorption on a series of adsorbents, such as agaroses substituted with ligands of increasing hydrophobicity, than through differential elution from a single adsorbent. Differential adsorption is carried out by leading the protein mixture through a "hydrophobicity gradient" consisting of interconnected columns of adsorbents of successively increasing hydrophobicity. Since in this manner a protein is bound by the adsorbent that provides the minimum degree of hydrophobicity required, the subsequently disconnected columns can be eluted with relatively mild eluants. With the aid of this procedure, mixtures of certain proteins, e.g., S7 gamma globulin and serum albumin, can readily be separated. Group separation of proteins in normal human blood serum also has been achieved. Further separation may be obtained on hydrophobic adsorbents of more gradually increasing hydrophobicity than have been used thus far. Protein fractions prepared by conventional procedures such as electrophoresis or differential precipitation, will be subjected to further separation on the basis of differences in hydrophobicity. It is expected that the introduction of the hydrophobic factor will present many new opportunities for the isolation of proteins from tissue extracts and body fluids. Special attention will be paid to the fractionation and purification of blood proteins.